A well-known result of climate change is the advance of spring, resulting in a mismatch between peak food availability and the arrival of migrant breeding birds. Researchers in the Netherlands and Sweden experimented with transporting birds north, resulting in an interesting impact on breeding success.
Earlier springs mean that the caterpillars on which migrant birds depend to raise successful broods now grow and pupate significantly earlier than they did a few decades ago. This means the food supply runs dry ever earlier and adult birds cannot supply enough caterpillars for their brood, resulting in dramatic crashes in breeding success.
Pied Flycatcher populations are suffering as the life cycle of defoliating caterpillars advances with warmer springs. Researchers have shown that transporting birds further north could give later generations a helping hand (Mark Leitch).
There have been signs that resident species can adapt to the spring's shifting schedule to some degree, but long-distance migrants have no cues on which to judge the arrival of spring in Europe from their wintering grounds in Africa.
Researchers in Sweden wanted to test whether birds could have better breeding success if they simply moved further north, where spring still arrives later. To do this, they trapped freshly arrived Pied Flycatchers in the Netherlands and drove them overnight to an area near Lund in Skåne, Sweden. In the morning, the birds were released into the forest where peak caterpillar availability comes around two weeks later than the birds' original destination. At a distance of around 600 km, the birds might be able to make the extra leg in just two nights of migration.
Jan-Åke Nilsson, biology researcher at Lund University in Sweden, said: "The birds that were given a lift from the Netherlands to Skåne synchronised very well with the food peak!"
He said the transported birds had better breeding success than Pied Flycatchers that had arrived naturally in the area, as well as those that remained in the Netherlands and were monitored.
The next spring, the offspring of the transported birds, which were ringed in the nest, were found to have made the migration to their fledging site in Skåne naturally, rather than migrating their parents' original destination in the Netherlands. They arrived earlier than other Pied Flycatchers in Sweden and went on to have boosted breeding success themselves.
Nilsson added: "The number of small birds, particularly migratory birds, has decreased drastically throughout Europe. By flying a little further north, these birds, at least in principle, could synchronise with their food resources and there is hope that robust populations of small birds can be maintained, even though springs are arriving ever earlier."
The potential conservation applications for struggling long-distance migrant species could be significant. Other species, such as Wood Warbler, are apparently suffering the same mismatch between migration timing and food availability as Pied Flycatcher.
Lamers, K P, Nilsson, J-A, Nicolaus, M, & Both, C. 2023 Adaptation to climate change through dispersal and inherited timing in an avian migrant. Nature Ecology & Evolution. DOI: https://doi.org/10.1038/s41559-023-02191-w